Memory array buried digit line

ABSTRACT

A method of forming a buried digit line is disclosed. Sacrificial spacers are formed along the sidewalls of an isolation trench, which is then filled with a sacrificial material. One spacer is masked while the other spacer is removed and an etch step into the substrate beneath the removed spacer forms an isolation window. Insulating liners are then formed along the sidewalls of the emptied trench, including into the isolation window. A digit line recess is then formed through the bottom of the trench between the insulating liners, which double as masks to self-align this etch. The digit line recess is then filled with metal and recessed back, with an optional prior insulating element deposited and recessed back in the bottom of the recess.

This application is a divisional of U.S. application Ser. No.11/036,163, filed on Jan. 14, 2005, the entirety of which is herebyincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of integrated circuitfabrication, specifically to the formation of memory arrays.

DESCRIPTION OF THE RELATED ART

Since the introduction of the digital computer, electronic storagedevices have been a vital resource for the retention of data.Conventional semiconductor electronic storage devices, such as DynamicRandom Access Memory (DRAM), typically incorporate capacitor andtransistor structures in which the capacitors temporarily store databased on the charged state of the capacitor structure. In general, thistype of semiconductor Random Access Memory (RAM) often requires denselypacked capacitor structures that are easily accessible for electricalinterconnection.

The capacitor and transistor structures are generally known as memorycells. The memory cells are arranged into memory arrays. The memorycells are addressed via a word line and a digit line, one of whichaddresses a “column” of memory cells while the other addresses a “row”of memory cells.

In many DRAM devices, the digit line is buried below the upper level ofthe substrate. One example of this is burying the digit line within theisolation trench. However, this can often involve several complicatedsteps. Furthermore, as integrated circuit designs become more dense, itbecomes more difficult to isolate a buried digit line within its trenchand to make contact with individual transistors in the array.

Thus, simpler and more reliable methods for forming, isolating, andcontacting buried digit lines are desired.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method is provided forforming an integrated circuit. The method includes forming an elongatedtrench between a first transistor active region and a second transistoractive region. An isolation element is deposited asymmetrically withinthe trench in contact with the second transistor active region. A bitline structure is deposited within the trench in direct contact with theactive region and the isolation element, wherein the isolation elementis positioned between the bit line structure and the second transistoractive region.

In accordance with another aspect of the invention, a method is providedfor forming a buried digit line. The method includes forming a trench ina substrate with a base and side walls. A first spacer is formed along afirst trench side wall and a second spacer along a second trench sidewall. The trench is filled with a first sacrificial material afterforming the first spacer and the second spacer. The second spacer isremoved to expose a portion of a base of the trench after filling thetrench with the first sacrificial material. The exposed first portion ofthe base of the trench is etched to form an isolation window having afirst depth. A first insulating liner is deposited along the firsttrench wall and the second insulating liner is deposited along thesecond trench side wall into the isolation window. A recess is formed inthe substrate by etching a second exposed portion of the base of thetrench between the first liner and the second liner to a second depth. Adigit line is then formed in the recess.

In accordance with another aspect of the invention, a method of forminga memory array is provided. The method includes forming an elongatedtrench having first and second sides in a substrate. An asymmetricisolation window is formed in the trench using sacrificial spacers,where the asymmetric isolation window is formed along the second side ofthe trench. Insulating spacers are deposited along the sides of thetrench and fill the insulation window. A digit line recess is formedbetween the insulating spacers in the substrate beneath the trench. Adigit line is formed in the digit line recess. The digit lineelectrically connects to a first memory cell on the first side and iselectrically isolated by the asymmetric isolation window from a secondmemory cell on the second side.

In accordance with another aspect of the invention, a computer memorystructure is provided. The structure includes a plurality of activeregions in a substrate, where the active regions are arranged in aplurality of columns. A trench in the substrate separates a first columnfrom a second column. A digit line in the trench directly contacts thefirst column. A filled asymmetric isolation window within the trenchseparates the digit line from the second column.

In accordance with another aspect of the invention, an integratedcircuit is provided, including a first elongated semiconductor ridge anda second elongated semiconductor ridge parallel to and spaced from thefirst ridge. The first and second ridges separated by a trench. Each ofthe first and second ridges serve as active areas for a plurality oftransistors along the lengths of the ridges. The trench includes aconductive line in continuous electrical contact with the first ridge.An insulating element within the trench separates the conductive linefrom the second ridge.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be better understood fromthe detailed description below and the appended drawings, which aremeant to illustrate and not to limit the invention, and in which:

FIG. 1 is a schematic, cross-sectional side view of a substrate withtrenches with a thin “pad oxide” grown over the surface of thesubstrate, a thicker layer of silicon nitride (Si₃N₄), and a photoresistmask in accordance with a starting point for preferred embodiments ofthe present invention.

FIG. 2 is a schematic, cross-sectional side view of the substrate ofFIG. 1 after spacers have been formed and the trench has been filledwith a sacrificial material.

FIG. 3 is a schematic, cross-sectional side view of the substrate ofFIG. 2 after one of the spacers has been removed and an etch into thesubstrate has been performed.

FIG. 4 is a schematic, cross-sectional side view of the substrate ofFIG. 3 with the remaining spacer and sacrificial material removed.

FIG. 5 is a schematic, cross-sectional side view of the substrate ofFIG. 4 after depositing insulating liners.

FIG. 6 is a schematic, cross-sectional side view of the substrate ofFIG. 5 after etching the substrate using the liners as a mask.

FIG. 7 is a schematic, cross-sectional side view of the substrate ofFIG. 6 after depositing an insulating material in the trench.

FIG. 8 is a schematic, cross-sectional side view of the substrate ofFIG. 7 after depositing and recessing a digit line material in thetrench.

FIG. 9 is a schematic, cross-sectional side view of the substrate ofFIG. 8 after depositing an insulating material in the trench and etchingback the insulating material.

FIG. 10 is a schematic, cross-sectional side view of the substrate ofFIG. 9 after forming transistor pillars and cell capacitors.

FIG. 11 is a schematic, cross-sectional side view of an array of memorycells formed in accordance with another embodiment of the invention.

FIG. 12 is a schematic, cross-sectional plan view of the array of memorycells taken along lines 12-12 of FIGS. 10 or 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment, a buried digit line is formed in a trenchbetween rows of transistors. After forming trenches, spacers are formedwithin each trench. A sacrificial material is deposited within thetrenches. One of the spacers is then removed from the trench, and thesubstrate below the removed spacer is etched to form an isolationwindow. After the isolation window is formed, the spacers andsacrificial material are removed. An insulating liner is formedconformally over the memory array. A spacer etch is then performed topreferentially etch horizontal surfaces. This exposes a portion of thetrench. The exposed bottom of the trench is preferably etched at thisstage to provide a recess in the substrate. If the insulating linerreaches the bottom of this recess, then the digit line can be depositeddirectly into the recess. Otherwise, an insulating layer is preferablydeposited into the trench before forming the digit line. An insulator isformed within the trench, and then etched back. Transistors andcapacitors are completed at positions between and above the trenches toform the memory cell.

Referring now to an embodiment illustrated in FIG. 1, a semiconductorsubstrate 10, such as bulk silicon, like a silicon wafer, is provided. Acap layer 15 may be formed over the substrate 10 in order to protect thesubstrate 10 from damage that could be caused during processing. The caplayer is preferably silicon nitride, but other insulating materials canalso be used. Preferably, the trenches are then masked using photoresist20 although other masking techniques can also be used.

In a first step, trenches are formed in the substrate 10. The trench canbe formed in a variety of methods. Preferably, an anisotropic dry etchprocess, such as a reactive ion etch process, is used to etch thetrenches. In a preferred embodiment the trench has a depth of betweenabout 1500 Å and 6000 Å, more preferably between about 2000 Å and 3000Å. The width of the trenches is preferably between about 100 Å and 2000Å, more preferably between about 350 Å (using a 0.04 μm process) and1000 Å (using a 0.100 μm process). An oxidation of the walls and base ofthe trench followed by an oxide etch step may also be performed in orderto smooth trench walls. Skilled practitioners will appreciate thattrenches can be formed in a variety of ways.

As seen in FIG. 2, after the trench is formed, a first set of spacers 22is formed on the walls of the trench. Preferably, a conformal liner ofspacer material is deposited over the array. The spacer material ispreferably silicon dioxide, but can also be other materials which can beselectively etched relative to the surrounding materials. A spacer etch,which preferentially removes horizontal layers relative to verticallayers, is then performed to expose a portion of the base of the trenchand leaving the spacers 22 along the sidewalls of the trench. Thespacers 22 preferably have a thickness of between about 50 Å and 600 Å,more preferably between about 100 Å and 300 Å, representing about ⅓ ofthe trench width.

After forming the spacers 22, a sacrificial material 25 is depositedover the array, filling the trenches. In a preferred embodiment, thesacrificial material 25 is polysilicon, but the sacrificial material canbe any material that can be selectively etched to the material of thespacer 22.

Referring now to FIG. 3, one of the spacers 22 along the sidewalls ofthe trench is removed. In a preferred embodiment, a photoresist mask 30is used during an etch of the sacrificial material 25 and one of thespacers 22. However, skilled practitioners will appreciate other maskingtechniques can be used. The exposed sacrificial material 25 is etchedthrough the mask before the spacer 22 is removed. This etch process canbe performed in distinct steps or in one etch step.

After one spacer 22 is removed, a portion of the trench floor is leftexposed. An etch process which will etch the substrate 10 selectively tothe sacrificial material 25 is then performed to form an isolationwindow or slot 35. Preferably, the isolation window 35 is asymmetric inthat it will contact one side of the digit line, but not the other. In apreferred embodiment, the isolation window extends between about 500 Åand 3000 Å below the trench floor, more preferably between about 1000 Åand 2000 Å.

As seen in FIG. 4, the mask and remaining sacrificial material andspacer material are preferably removed after forming the isolationwindow 35.

In FIG. 5, a second set of spacers is formed. First, an insulating layer40 is conformally deposited over the array and the cap layer 15. Theinsulating layer 40 is preferably silicon nitride, but otherelectrically insulating materials can also be used. The insulating layerpreferably fills the isolation window 35 with a lower insulating layer.Preferably the insulating layer 40 has a thickness along the sidewallsof between about 60 Å and 600 Å, more preferably between about 100 Å and200 Å.

After the conformal insulating layer is deposited, another spacer etchis performed to preferentially etch the horizontal surfaces of theinsulating layer 40 and expose a second portion of the trench floor.This etch leaves remaining portions of the insulating layer 40 on thetrench side walls in the form of insulating spacers that extend into theisolation window 35.

An etch process selectively etches the substrate material relative tothe materials selected for the cap layer 15 and the insulating layer 40to recess the exposed portion of the trench floor to form a lower recess45 in the substrate 10. In a preferred embodiment, this etch processetches between about 10 Å and 3000 Å of the substrate 10, morepreferably between about 200 Å and 2500 521 . The insulating layer 40along the sidewalls and the lower insulating layer in the isolationwindow 35 insulate the surrounding substrate. As can be seen from FIG.6, one entire side of the lower recess 45 is exposed to the substrate10, while the other side of the lower recess 45 is partially bounded bythe isolation window 35.

In FIG. 7, an insulating material 50 is deposited into and recessed backin the lower recess 45 so that only one sidewall of the recess iselectrically exposed to the digit line which will be formed within thelower recess 45. In a preferred embodiment, the insulating material 50has a thickness of between about 100 Å and 2000 Å, more preferablybetween about 500 Å and 800 Å. In order to fully isolate the selectedside of the lower recess, the thickness of the insulating material 50 isgreater than the distance between the bottom of the lower recess and thebottom of the isolation window 33. In other words, the insulatingmaterial 50 overlaps with the insulating lay 40 to completely isolatethe right side of each trench.

As illustrated in FIG. 8, once one side of the lower recess 45 iscompletely electrically isolated, a conductive digit line 55 is formedwithin the lower recess 45. Preferred materials for the digit line 55include metals and metal alloys. Exemplary materials include titaniumnitride, titanium, and tungsten. Preferably, the digit line 55 has avertical thickness of between about 100 Å and 2000 Å, more preferablybetween about 300 Å and 600 Å.

In one embodiment, a multi-level digit line 55 is formed with layers ofseveral materials. In a preferred embodiment, a lower layer of titaniumis first deposited, serving as an adhesion layer, followed by a middlelayer of titanium nitride, serving as a conductive barrier, and an upperlayer of tungsten fills the remainder of the trench. The thickness ofthe middle barrier layer is preferably between about 20 Å and 500 Å,more preferably between about 40 Å and 80 Å. The thickness of the loweradhesion layer is preferably between about 10 Å and 600 Å, morepreferably between about 100 Å and 300 Å. The thickness of the upperlayer is preferably between about 100 Å and 1500 Å, more preferablybetween about 300 Å and 600 Å. Each such deposition can line the lowerrecess 45, thus extending over the trench sidewalls.

As seen in FIG. 9, after depositing and recessing the digit line 55, thetrench is filled with an insulating material 60. In a preferredembodiment the insulating material is an oxide, such as a tetraethylorthosilicide (TEOS) oxide or a spin-on oxide. The insulating materialis then preferably etched back or planarized, through a process such aschemical mechanical polishing (CMP). In a preferred embodiment, theinsulating material fills the trench, and is typically overflows bybetween about 50 Å and 2000 Å, more preferably between about 300 Å and600 Å, before CMP or other etch back.

In a preferred embodiment, the buried digit lines 55 are then used toform a DRAM array. An exemplary array is seen after several stages ofprocessing in FIG. 10. Several DRAM process can be used to form thememory array. One example process is found in U.S. patent applicationSer. No. 10/934,621 of Tang, et. al, the disclosure of which is herebyincorporated herein by reference.

In the illustrated embodiment of FIG. 10, a transistor pillar 65 isformed on the substrate between the trenches. In a preferred embodiment,the pillars are epitaxial silicon, though in other arrangements thepillars can be etched from a substrate. A gate oxide 70 is then formedon the sides of the transistor pillars 65. In a preferred embodiment, asource region is formed along ridges between the trenches, preferablycontacting the transistor pillar. In preferred embodiments, the drain isformed at the top of the transistor pillar 65 and the body of the pillardefines the transistor channel. A word line 75 is formed betweenneighboring cells. In a preferred embodiment, the word line 75 is aconductive polysilicon and may include strapping self-aligned silicide.While not apparent from the illustrated cross-section, a plurality ofword lines are formed in a crossing pattern with the bit lines. In apreferred arrangement, each word line surrounds a row of transistors andserves as a gate electrode for each of the transistors in the row. Aninsulating layer 80 is deposited over the word lines 75. The top of thetransistor pillar 65 is then exposed to form electrical contact to anoverlying stacked capacitor. In a preferred embodiment, the capacitorelectrode is a container capacitor. A bottom electrode 90 is formedelectrically connected to the transistor pillar 65. It will beunderstood that as intermediate contact plug can be employed between thepillar 65 and the bottom electrode 90. In a preferred embodiment, thebottom electrode 90 comprises a conductive metal or metal alloy. Acapacitor dielectric (not pictured) is then formed over the bottomelectrode. A top electrode is then formed the dielectric. In a preferredembodiment the top electrode is a common reference electrode for thewhole array.

An exemplary process flow for the illustrated vertical surround gate(VSG) transistor is disclosed in U.S. application Ser. No. 10/934,621,filed Sep. 2, 2004, the disclosure of which is incorporated by referenceherein. The skilled artisan will readily appreciate, however, that theburied bit line processes and structures disclosed herein are useful fora number of different transistor and memory array designs.

Thus, in a preferred embodiment illustrated in FIG. 10, the digit line55 is electrically connected to the substrate 10, and particularly tothe transistor sources, on one side of the trench, and isolated by theisolation liner 40 on the other side (right side in FIG. 10).Preferably, the bottom of the insulating liner 40 in the isolationwindow 35 extends below or even with the bottom of the digit line 55. Inthe illustrated embodiment, an insulator 50 is formed beneath the digitline 55 within the lower recess 45. The digit line 55 is preferablyisolated from above by an insulating material 60.

In a preferred embodiment, vertical transistors are formed between thetrenches. The vertical transistors include transistor pillars 65 overthe substrate 10. Preferably, a plurality of transistor pillars 65 areformed on a ridge running parallel between the trenches in the dimensioninto and out of the paper. A gate oxide 71 surrounds the sidewalls ofthe transistor pillar 65. Preferably, a word line 75 serves as the gateelectrode for each of a plurality of transistors in a row. An insulatinglayer 80 is formed over the word line 75. A bottom container capacitorelectrode 90 is formed over each transistor pillar. A capacitordielectric and top electrode is preferably formed over each of theelectrodes. These structures are arranged in a memory array. The numberof cells, trenches, and digit lines may vary based upon the desiredcapacity of the memory array.

With reference to FIG. 11, in another embodiment, the isolation window35a formed using the spacers as is extended deeper into the substrate 10than in the embodiment of FIG. 4. Preferably, the isolation window isextended below the bottom of the subsequently formed lower recess 45 a.As described above, the isolation window 35 a is filled with theinsulating layer 40 a. The subsequent lower recess 45 a extends toapproximately the same depth or less deep than the isolation window 35a. Preferably the bottom of the isolation window 35 a is 100 Å to 2000 Åbelow the bottom of the digit line 55 a, more preferably the bottom ofthe isolation window 35 a is 500 Å to 800 Å below the bottom of thedigit line 55 a. The lower insulating material 50 of FIG. 7 can thus beomitted, saving the deposition and recess steps therefore. Accordingly,the digit line 55 a is deposited directly into the lower recess 45 a.

In the resulting structure, the bottom of the insulating material 40 ain the isolation window 35 a preferably extends below the digit line 55a or is co-extensive with the bottom of the digit line 55 a. On theother side of the trench, the top edge of the digit line 55 a isisolated from the transistor channel by the isolation liner 40 a.

As best seen from the cross-sectional plan of FIG. 12, the resultantburied digit line 55 or 55 a directly contacts the ridge of thesubstrate 10 along which a column of source regions 95 are formed. Thebit digit line 55 or 55 a is in continuous contact with the ridge ofsubstrate material 10, such that no independent bit line contactstructure is required. Rather, the digit line 55 or 55 a intermittentlycontacts source regions along its length. It will be understood that thesource regions extend upwards to the surface of the substrate 10, whereepitaxial pillars extend upwards and form the channel regions of thetransistors. Orthogonal to the digit lines 55 or 55 a are a plurality ofword lines 75, shown in dotted lines in FIG. 12, overlapping a row oftransistors and surrounding the pillar channel regions to definevertical surround gate (VSG) transistors. On one side of the digit linestructures 55 or 55 a, the insulating layer 40 or 40 a electricallyseparates the digit line 55 or 55 a from the next adjacent ridge ofsubstrate material 10.

Advantageously, because the digit line 55 or 55 a directly contacts thesubstrate ridge 10 in the source regions, no separate contact structureis required. Not only does this save the additional process steps forforming a contact structure, but also save the additional space thatwould be otherwise required for making separate bit line contacts.

It will be appreciated by those skilled in the art that variousomissions, additions and modifications may be made to the methods andstructures described above without departing from the scope of theinvention. All such modifications and changes are intended to fallwithin the scope of the invention, as defined by the appended claims.

1. A method of forming a buried digit line comprising: forming a trenchin a substrate, wherein the trench has a base, a first trench side wall,and a second trench side wall; forming a first spacer along the firsttrench side wall and a second spacer along the second trench side wall;filling the trench with a first sacrificial material after forming thefirst spacer and the second spacer; removing the second spacer to exposea first portion of the base of the trench after filling the trench;etching the exposed first portion of the base of the trench to form anisolation window with a first depth; depositing a first insulating lineralong the first trench side wall and a second insulating liner along thesecond trench side wall and into the isolation window; forming a recessin the substrate by etching a second exposed portion of the base of thetrench between the first insulating liner and the second insulatingliner to a second depth; and forming a digit line in the recess.
 2. Themethod of claim 1, wherein forming the digit line comprises forming thedigit line with a bottom approximately at the second depth.
 3. Themethod of claim 1, wherein forming the digit line comprises forming aplurality of layers.
 4. The method of claim 1, wherein forming the firstand second insulating liners comprises forming nitride liners.
 5. Themethod of claim 1, wherein forming the first and second spacerscomprises forming oxide spacers.
 6. The method of claim 1, wherein thefirst insulating liner has a thickness along the first trench side wallof between about 100 Å and 300 Å.
 7. The method of claim 1, furthercomprising depositing an insulating material in the recess prior toforming the digit line.
 8. The method of claim 7, wherein depositing theinsulating material comprises depositing silicon oxide.
 9. The method ofclaim 7, wherein a bottom of the digit line is above the first depth.10. The method of claim 1, wherein forming the trench in the substratecomprises forming a trench between a first transistor active region anda second transistor active region.
 11. The method of claim 10 whereinthe first transistor active region comprises a plurality of transistorsource regions in electrical contact with the digit line.
 12. The methodof claim 11, further comprising forming a plurality of transistorpillars above the source regions and defining a drain region at a top ofeach transistor pillar.
 13. The method of claim 12, further comprisingforming a gate dielectric along a side wall surface of each of thetransistor pillars.
 14. The method of claim 13, further comprisingforming a word line surrounding each transistor pillar.
 15. The methodof claim 14, further comprising forming a stacked capacitor over and inelectrical contact with the drain region of each transistor pillar. 16.The method of claim 10, wherein the digit line continuously contacts thefirst transistor active region along a trench axis.
 17. The method ofclaim 1, wherein forming a digit line at the bottom of the recesscomprises depositing conductive material in direct contact with thesubstrate at the bottom of the trench.
 18. The method of claim 1,wherein the recess extends below a surface of the substrate farther thanthe isolation window extends below the surface of the substrate.
 19. Themethod of claim 1, wherein the isolation window extends below a surfaceof the substrate farther than the recess extends below the surface ofthe substrate.
 20. The method of claim 1, wherein the trench has a widthbetween the first and second trench side walls of between about 350 Åand 1000 Å.